1. Field of the Invention
This invention relates generally to a battery cooling system for a vehicle and, more particularly, to a battery cooling system for a fuel cell hybrid vehicle, where the battery cooling system uses the exhaust gas flow from a fuel cell stack to create a partial vacuum that draws air through the battery system from the passenger compartment of the vehicle.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The dynamic power of a fuel cell system is limited. Further, the time delay from system start-up to driveability and low acceleration of a fuel cell vehicle may not be acceptable. During a drive cycle, the stack cell voltage varies because the variable driver power request follows the stack polarization curve. The voltage cycles can decrease the stack durability. These drawbacks can be minimized by using a high voltage battery in parallel with the fuel cell stack. Algorithms are employed to provide the distribution of power from the battery and the fuel cell stack to meet the requested power.
For the reasons discussed above, some fuel cell vehicles are hybrid vehicles that employ a rechargeable supplemental power source in addition to the fuel cell stack, such as a DC battery or a super capacitor (also referred to as an ultra-capacitor or double layer capacitor). The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to a traction motor and other vehicle systems through a DC voltage bus line for vehicle operation. The battery provides the supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to meet the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery through the DC bus line.
In one known fuel cell vehicle hybrid design, the battery is part of a battery system enclosed within a housing or compartment, where the system includes a plurality of battery modules, each having a number of battery cells. The battery system further includes various power control modules, electronic control circuits, etc., enclosed within the compartment. In this particular design, it is typically necessary to cool the battery modules because they generate a significant amount waste heat that could reduce the performance and life of the battery system.
It is known in the fuel cell art to use the cooling sub-system that cools the fuel cell stack to also cool the battery system. For example, flow channels may be provided at appropriate locations in the battery system compartment that are plumbed to the coolant loop through which a cooling fluid flows that cools the fuel cell stack. The cooling sub-system typically will include a radiator and a fan that reduces the temperature of the cooling fluid before it is returned to the fuel cell stack to collect the stack waste heat. However, this type of liquid cooling system for the battery system has a number of drawbacks in that it is fairly complex to provide the necessary pipes and such to direct the cooling fluid to the battery compartment. Further, during high load conditions, the ability of the radiator and cooling fan to cool the cooling fluid is reduced, which may not be adequate to cool the battery system to the desired temperature, thus requiring a larger radiator and fan to cool both the fuel cell stack and the battery system under these conditions. Also, the normal operating temperature of the fuel cell stack, typically between 60° C. and 80° C., is generally too high for proper battery system cooling. Generally, it is desirable to maintain the temperature of the battery system below 40° C.
It is also known in the art employ a separate thermal sub-system for the cooling the battery system that uses air as the cooling medium. In one particular design, a fan is employed to draw air from the passenger compartment of the vehicle, which is typically at a comfortable temperature, and drive the air through the battery compartment so that it flows around the battery modules. The heated air that flows through the battery compartment is then exhausted to the environment. The air cooled thermal sub-system is typically not able to provide as great amount of cooling as a liquid cooled thermal sub-system, but it is less complex and easier to implement. However, the fan used in the air-cooled battery thermal sub-system requires power to drive the fan, and suitable packaging space. Therefore, it is desirable to eliminate the fan to reduce the weight, packaging space and power required by the fuel cell vehicle.